The FSB said the suspect had been “guaranteed” a reward of $100,000 and permission to move to the European Union in exchange for killing Kirillov.
It added that on Ukraine’s instructions, he arrived in Moscow and received a homemade explosive device.
He placed the explosive device on an electric scooter, which he parked at the entrance to the residential building where Kirillov lived, the FSB said.
He then rented a car to monitor Kirillov’s residence and also installed a camera livestreaming a video feed from the site to his handlers in the Ukrainian city of Dnipro.
When they saw Kirillov emerge from the house, the suspect detonated the bomb, the statement said.